The other day I overheard a tidbit of sex education being taught to two and a half year old Tommy by his five year old sister, Ellen. It went like this: "Boys can't have babies because their penises are in the way."
I didn't intervene. For the time being, I figure her explanation covers it to the extent necessary for their ages. If it comes up again in ten or fifteen years, however, I will explain that boys can't have babies for other reasons. I will also tell her that there is, indeed, much boys cannot do because their penises are in the way though, including but not limited to multitasking, concentrating when boobs are nearby (or rock hard pecs in some instances), asking for directions and buying tampons. And when Tommy finds it impossible to do those things when called upon to do them, I will say. "Blame your penis, it's probably in the way."